As a user moves through a radio coverage area, severe amplitude fades are encountered at half wave length intervals. If the depth of these amplitude fades exceed the threshold level for reliable communications of a receiver, communications will fail. The effects of multipath fading are even more detrimental when transmitting high speed digital information over a fading channel.
Two common techniques for combatting multipath fading are Forward Error Correction (FEC) and Data Redundancies. FEC techniques append a block of message reconstruction information to each transmission. Should the transmitted message be received with errors, the reconstruction information enables reconstruction of the message at the receiver end. The problems with this approach include taxation of the processing power of a receiver due to the computationally intensive FEC techniques, and the reduction in system throughput caused by the additional FEC code added to each transmission.
Another method for reducing multipath fading uses data redundancies. In this simple scheme, each message is transmitted multiple times. A majority voting process at the receiving unit creates one good message out of several messages containing errors. This form of multipath interference reduction is less computationally intensive than FEC techniques and requires less processing time. However, multiple transmissions still cause a severe reduction in system throughput.
Both FEC and data redundancy techniques share the same two fundamental deficiencies in dealing with the destructive interference caused by multipath fading. First, both techniques are pro-active. Instead of eliminating multipath fading, the techniques focus on retrieving data once the damage has occurred. The other deficiency is that if a user is stationary and in a deep fade condition, no amount of FEC or redundant transmissions will enable communication.
By contrast, diversity reception is a method of reducing the destructive effects of multipath fading by addressing the problem more directly. Presently existing diversity reception techniques utilize multiple antennas or multiple receivers to recover modulated signals and require no FEC or data redundancy techniques. However, both the multiple antenna and multiple receiver techniques have been largely unsuccessful for a number of reasons.
In the multiple antenna scheme, a receiver has a plurality of antennas for receiving signals. In most multiple antenna schemes, a signal level threshold is set, and when a received signal from a particular antenna falls below the threshold, the receiver switches to another antenna in hopes of receiving a better signal. These schemes have been unsuccessful because of the impossibility of predicting the signal impressed on the unmonitored antennas. Thus, when switching from one antenna to another, no guaranty exists that another antenna will provide a better signal.
In a multiple receiver scheme, a plurality of receivers are available to supply recovered modulated signals. A composite received signal is assembled from each received signal using a combination or selection algorithm. This type of technically complex scheme has been largely unsuccessful due to the inadequacies of high speed RF signal level quantification circuitry and the inadequacies of the selection algorithm. Another problem with multi-receiver systems are the damaging transients resulting from high speed switching between phase discontinuous signals. Therefore, a need has arisen for a diversity reception system, capable of successfully reducing destructive interference caused by multipath fading.